A magnetostrictive material is a material whose shape changes as a function of applied magnetic field. For example, if a rod of magnetostrictive material of length l is subject to an applied magnetic field H, its length will decrease or increase, with the change in length Δl saturating at the value Δl=λsl, where λs is the magnetostrictive coefficient of the material, also referred to as the saturation magnetostriction. A common magnetostrictive material is nickel for which λs equals about −40 parts per million (ppm) or −40×106. When a magnetic field is applied to a magnetostrictive material, it will expand or contract, such that a positive λs refers to expansion and a negative λs refers to contraction. The material currently having the largest known magnetostrictive coefficient is an alloy of terbium, dysprosium and iron, commonly referred to as Terfenol, which possesses λs>1000 ppm. This is approximately 100 times larger than nickel. A magnetostrictive coefficient in this range, often referred to as giant magnetostriction, may be useful for many technological applications, such as actuation and sensing. However, the Terfenol alloy is brittle and expensive.
Significant effort has recently been devoted to the fabrication of composites of Terfenol and other materials, including polymers and metals, in an effort to combine the good magnetostrictive properties of Terfenol with a matrix material having more robust mechanical properties. One such effort is disclosed in Pinkerton et al. U.S. Pat. No. 5,993,565, which describes composites of Terfenol with a metal matrix of aluminum, copper, iron, magnesium or nickel. In these composites, the magnetostrictive coefficient λs is reduced by approximately the volume fraction of the magnetostrictive phase. For example, a composite containing 50% Terfenol would have a magnetostrictive coefficient approximately half that of bulk Terfenol.
Other efforts have recently been directed to composites of Terfenol in an epoxy resin matrix. Highly metal-filled epoxy resins are generally rigid but brittle materials and may be subject to failure even under mild conditions. However, recent theoretical models, such as that disclosed in Nan et al. 60 Physical Review B, 6723 (1999), suggest that the magnetostrictive coefficient λs a composite, rather than having a directly proportional dependence on magnetostrictive particle content, depends in part on the elastic properties of the matrix material, and in particular, for Terfenol/epoxy composites, large magnetostriction may still be obtained at lower volume filling fractions of Terfenol due to the softness of epoxy. This theoretical work suggests that further investigation into magnetorestrictive composites may yield improved commercial products than those currently available.
There is thus a need to provide a less rigid magnetostrictive composite having large magnetostrictive properties and good mechanical properties, such that the material can be used in aggressive environments, such as automotive applications and the like.